The present invention relates to a composition which is capable of imparting non-Newtonian, pseudoplasticity and water loss controlling properties to aqueous systems. More particularly, the present invention relates to the formation of an improved water-based, clay-free drilling fluid containing the subject composition and to an improved method of drilling bore holes into subterranean formations using said drilling fluid.
In normal well drilling operations in which a well is drilled by a rotary method, the well bore hole is generally filled with a drilling fluid or mud which is circulated therein. Drilling fluids are usually pumped down through the drill stem of the rotary rig, circulated around the drill bit and returned to the surface through the annular passage between the drill stem and well wall. These drilling fluids perform a number of functions including lubricating the drill stem and bit, cooling the bit, carrying the cuttings from the bit up the bore hole to the surface where the cuttings may be separated and disposed of, and providing a hydrostatic head against the walls of the well to contain downhole geopressure.
A primary requisite of a satisfactory drilling fluid is its ability to readily circulate and flow, that is, to have low viscosity, under the high sheer conditions which are present in the environs of the drill bit while, at the same time, being capable of having and maintaining sufficient viscosity to be capable of efficiently carrying the bit cuttings to the surface.
A typical fluid composition contains various agents to impart needed properties to the fluid at different stages in the drilling operation. The drilling fluid must also be capable of inhibiting the amount of liquid, normally water, which is lost into the porous strata through which the bore hole traverses. The loss of liquid causes the formation and build-up of a cake deposit which, after a period of time, can cause sticking of the drill pipe and stoppage of the drilling operation. The fluid must, therefore, be of a nature which permits minimum loss into the porous strata. Agents which impart such property are conventionally termed "water loss controllers".
The drilling fluid must also be capable of exhibiting the above-described water-loss and pseudoplastic properties under changing composition and environmental conditions encountered during the drilling operation due to the fact that the bore hole traverses various strata such as shales, clay, etc., and the cuttings of these materials become dispersed in the fluid media. The drilling fluid components should be substantially stable to the presence of various calcium compounds and to sodium chloride which may be present in the fluid from the soil strata with which it is in contact and/or due to the use of salt water having calcium or sodium salts therein used in forming the drilling fluid.
It is desired that the drilling fluid components should be stable and functional at elevated temperature. It is well known that as the bore hole increases in depth the temperatures encountered are substantially above that found at the earth's surface. Further, heat is generated by frictional forces on the drill bit. It is, therefore, desired that components used in forming drilling fluids be stable with respect to varying elevated temperature conditions.
A wide variety of drilling fluids have been used, including aqueous-based liquids, hydrocarbon based liquids, air and other gases, mist, foams and the like. Since great volumes of drilling fluids are required for the purposes of providing a cooling medium for the rotary bit and a means of carrying off the drilled particles, most of the conventional fluids used have been based on water. Water alone being a Newtonian fluid, does not have the needed capability to efficiently carry the drilled particles from the bore hole to the surface nor the capacity of inhibiting loss of fluid into adjacent porous strata.
It is a widely held and accepted theory that the viscosities suitable for creating a particle carrying capacity in the fluid can be achieved with the drilling fluid having pseudoplastic properties. For example, the drilling fluid must be capable of having a low viscosity under the high shear rates such as encountered at the drill bit, yet have the ability to increase in viscosity (and, therefore, particle holding power) under decreasing shear rates encountered in its upward movement through the annulus.
In order to obtain the requisite pseudoplastic properties, it has been thought desirable to use clay or colloidal clay bodies such as bentonite. As a result, the drilling fluids have been usually referred to as "muds". The use of clay based drilling fluids has provided the means of initially meeting the two basic requirements of drilling fluids, i.e., cooling and particle removal. However, the clay-based drilling fluids are highly unstable when they come in contact with various salts found in drilled earth formations.
Materials which have come into expanding use to impart rheological properties to drilling compositions are xanthan gums such as are described in U.S. Pat. Nos. 3,198,268; 3,208,526; 3,251,147; 3,243,000; 3,307,016; 3,319,715 and 3,988,246. These materials have been found to cause aqueous solutions, such as drilling fluids, to exhibit pseudoplastic properties under varying low shear rates. However, these materials, whether used alone or in combination with other additives, present the problem of being irreversibly degraded by the elevated temperatures often encountered during conventional drilling operations and thereby require continuous supplementation of material. The high cost of the xanthan gums, and the high rate of degradation, limit their usefulness to specialized operations.
Prior utilization of hydroxides or hydrated metal oxides of amphoteric metals in well treating fluids have involved properties distinctly different from the properties required for a drilling fluid as described herein. For example, U.S. Pat. Nos. 3,614,985 and 3,815,681 describe a process for plugging a subterranean reservoir by permeating its pores with a solution containing an amphoteric metal salt and a pH increasing reactant to cause precipitation in the pores. U.S. Pat. No. 3,603,399 describes a process for treating a water sensitive formation by permeating its pores with a hydroxy-aluminum solution which is a clear and relatively non-viscous solution. In each of such prior well treating processes, it has been important that the solution have relatively low viscosity and high filter loss to ensure that the solution penetrates into the matrix or pores of the reservoir. In contrast, in a drilling fluid, it is important that the fluid be capable of having a high viscosity over the major region of its use (the annular region of the drill stem), exhibit a low viscosity at the drill bit site and be capable of not penetrating the formation and thereby leaving a filter cake over the entire bore hole. U.S. Pat. No. 3,860,070 describes a well completion or fracturing fluid containing an amphoteric metal salt and a base in a ratio to make the final solution strongly acidic in order to form a thickened fluid suitable as a fracturing fluid. Such fluids can not be used satisfactorily in a drilling operation due to their corrosive nature with respect to the metal drilling equipment. None of the various well treating fluids described in the above references would be suitable for their intended purpose if they contained a fluid loss agent.
The viscosity of a drilling fluid has been relied upon as a mode of aiding in fluid loss control with little success especially when drilling into and through porous substrates. To enhance the control, various agents have been added. For example, in U.S. Pat. No. 3,032,498 a cyanoethylated starch was described as a water loss controller when used in combination with a clay-based mud. U.S. Pat. No. 3,988,246 describes an esterified or etherified starch as a water loss controlling agent which is compatible with a xanthan gum based drilling mud. Other starches have been employed in clay free muds under limiting temperature conditions as starches are known to be temperature sensitive.
There is a general need for a composition which is capable of imparting both pseudoplastic and water loss controlling properties to aqueous compositions such as drilling fluid compositions. The composition should be stable to varying conditions and temperatures commonly encountered in drilling operations and must be easily produced at low cost to aid in the economics of drilling operations.